It is important for high performance tires and race tires to have excellent traction characteristics. In fact, race drivers frequently attribute winning or losing automobile races to their tires. For instance, superior dry traction characteristics allow drivers to go faster, giving them an edge over other drivers having tires that exhibit inferior traction characteristics. Durability can also be important in racing. For example, it is highly advantageous for a set of tires to be durable enough to finish a race without being replaced which, of course, eliminates the need for changing tires during a pit stop. It is, of course, always advantageous for high performance automobile tires to have good durability for increased tire life.
Tire tread compounds for race tires and high performance tires are formulated to attain the best possible combination of traction characteristics and durability. The traditional problem associated with this approach is that traction characteristics and skid resistance characteristics are generally compromised to attain better durability. In order to balance these two inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in high performance tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubbery material for high performance automobile tire treads. High levels of aromatic processing oils are also normally included in high performance tire tread formulations to increase dry traction characteristics (see U.S. Pat. No. 4,861,131). However, high levels of aromatic oils in tread compounds typically reduce the durability of the tire. Thus, the ability to improve the traction characteristics of a tire tread without sacrificing durability is often elusive.
U.S. Pat. No. 4,843,120 discloses that tires having improved performance characteristics can be prepared by utilizing rubbery polymers having multiple glass transition temperatures as the tread rubber. These rubbery polymers having multiple glass transition temperatures exhibit a first glass transition temperature which is within the range of about xe2x88x92110xc2x0 C. to xe2x88x9220xc2x0 C. and exhibit a second glass transition temperature which is within the range of about xe2x88x9250xc2x0 C. to 0xc2x0 C. According to U.S. Pat. No. 4,843,120, these polymers are made by polymerizing at least one conjugated diolefin monomer in a first reaction zone at a temperature and under conditions sufficient to produce a first polymeric segment having a glass transition temperature which is between xe2x88x92110xc2x0 C. and xe2x88x9220xc2x0 C. and subsequently continuing said polymerization in a second reaction zone at a temperature and under conditions sufficient to produce a second polymeric segment having a glass transition temperature which is between xe2x88x9220xc2x0 C. and 20xc2x0 C. Such polymerizations are normally catalyzed with an organolithium catalyst and are normally carried out in an inert organic solvent.
U.S. Pat. No. 5,137,998 discloses a process for preparing a rubbery terpolymer of styrene, isoprene and butadiene having multiple glass transition temperatures and having an excellent combination of properties for use in making tire treads which comprises: terpolymerizing styrene, isoprene and 1,3-butadiene in an organic solvent at a temperature of no more than about 40xc2x0 C. in the presence of (a) at least one member selected from the group consisting of tripiperidino phosphine oxide and alkali metal alkoxides and (b) an organolithium compound.
U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outer circumferential tread where said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight rubber (phr), (A) about 10 to about 90 parts by weight of a styrene, isoprene, butadiene terpolymer rubber (SIBR) and (B) about 70 to about 30 weight percent of at least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadiene rubber wherein said SIBR rubber is comprised of (1) about 10 to about 35 weight percent bound styrene, (2) about 30 to about 50 weight percent bound isoprene and (3) about 30 to about 40 weight percent bound butadiene and is characterized by having a single glass transition temperature (Tg) which is in the range of about xe2x88x9210xc2x0 C. to about xe2x88x9240xc2x0 C. and, further, the said bound butadiene structure contains about 30 to about 40 percent 1,2-vinyl units, the said bound isoprene structure contains about 10 to about 30 percent 3,4-units, and the sum of the percent 1,2-vinyl units of the bound butadiene and the percent 3,4-units of the bound isoprene is in the range of about 40 to about 70 percent.
U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubber which is particularly valuable for use in making truck tire treads which exhibit improved rolling resistance and tread wear characteristics, said rubber being comprised of repeat units which are derived from about 5 weight percent to about 20 weight percent styrene, from about 7 weight percent to about 35 weight percent isoprene and from about 55 weight percent to about 88 weight percent 1,3-butadiene, wherein the repeat units derived from styrene, isoprene and 1,3-butadiene are in essentially random order, wherein from about 25 percent to about 40 percent of the repeat units derived from the 1,3-butadiene are of the cis-microstructure, wherein from about 40 percent to about 60 percent of the repeat units derived from the 1,3-butadiene are of the trans-microstructure, wherein from about 5 percent to about 25 percent of the repeat units derived from the 1,3-butadiene are of the vinyl-microstructure, wherein from about 75 percent to about 90 percent of the repeat units derived from the isoprene are of the 1,4-microstructure, wherein from about 10 percent to about 25 percent of the repeat units derived from the isoprene are of the 3,4-microstructure, wherein the rubber has a glass transition temperature which is within the range of about xe2x88x9290xc2x0 C. to about xe2x88x9270xc2x0 C., wherein the rubber has a number average molecular weight which is within the range of about 150,000 to about 400,000, wherein the rubber has a weight average molecular weight of about 300,000 to about 800,000 and wherein the rubber has an inhomogeneity which is within the range of about 0.5 to about 1.5.
U.S. Pat. No. 5,405,927 discloses a pneumatic truck tire having an outer circumferential tread wherein said tread is a sulfur-cured rubber composition comprised of, based on 100 parts by weight of rubber, (a) from about 25 to about 75 parts of an isoprene-butadiene rubber, said rubber being comprised of repeat units which are derived from about 20 weight percent to about 50 weight percent isoprene and from about 50 weight percent to about 80 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order, wherein from about 3 percent to about 10 percent of the repeat units in said rubber are 1,2-polybutadiene units, wherein from about 50 percent to about 70 percent of the repeat units in said rubber are 1,4-polybutadiene units, wherein from about 1 percent to about 4 percent of the repeat units in said rubber are 3,4-polyisoprene units, wherein from about 25 percent to about 40 percent of the repeat units in the polymer are 1,4-polyisoprene units, wherein the rubber has a glass transition temperature which is within the range of about xe2x88x9290xc2x0 C. to about xe2x88x9275xc2x0 C. and wherein the rubber has a Mooney viscosity which is within the range of about 55 to about 140; and (b) from about 25 to about 75 parts of natural rubber.
The traction characteristics of high performance tires and race tires can be improved without compromising durability by utilizing the technique of this invention. This approach involves substituting about 4 phr (parts per hundred parts by weight of rubber) to about 40 phr of liquid isoprene-butadiene rubber for a portion of the aromatic oil included in the tread rubber formulation. The liquid isoprene-butadiene rubber employed in the tread rubber formulations of this invention typically has a number average molecular weight which is within the range of about 3,000 to about 50,000 and a glass transition temperature which is within the range of about xe2x88x9250xc2x0 C. to about 20xc2x0 C. The tire tread rubber compounds of this invention will also normally contain about 20 phr to about 80 phr of an aromatic process oil and about 35 phr to about 130 phr of a filler.
This invention more specifically discloses a liquid isoprene-butadiene polymer which is comprised of repeat units which are derived from about 5 weight percent to about 95 weight percent isoprene and from about 5 weight percent to about 95 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order, wherein the liquid isoprene-butadiene polymer has a low number average molecular weight which is within the range of about 3,000 to about 50,000 and wherein the liquid isoprene-butadiene polymer has a glass transition temperature which is within the range of about xe2x88x9250xc2x0 C. to about 20xc2x0 C.
The subject invention further reveals a tire tread compound which is comprised of (a) a rubbery elastomer, (b) about 20 phr to about 80 phr of an aromatic process oil, (c) about 35 phr to about 130 phr of a filler and (d) about 4 phr to about 40 phr of a liquid isoprene-butadiene rubber having a number average molecular weight which is within the range of about 3,000 to about 50,000 and a glass transition temperature which is within the range of about xe2x88x9250xc2x0 C. to about 20xc2x0 C.
The subject invention further reveals a tire which is comprised of a generally toroidal-shaped carcass with an outer circumferential tread, two spaced beads, at least one ply extending from bead to bead and sidewalls extending radially from and connecting said tread to said beads; wherein said tread is adapted to be ground-contacting; wherein the tread is comprised of (a) a rubbery elastomer, (b) about 20 phr to about 80 phr of an aromatic process oil, (c) about 35 phr to about 130 phr of a filler and (d) about 4 phr to about 40 phr of a liquid isoprene-butadiene rubber having a number average molecular weight which is within the range of about 3,000 to about 50,000 and a glass transition temperature which is within the range of about xe2x88x9250xc2x0 C. to about 20xc2x0 C.
The IBR of this invention is synthesized by solution polymerization. Such solution polymerizations will normally be carried out in a hydrocarbon solvent which can be one or more aromatic, paraffinic or cycloparaffinic compounds. These solvents will normally contain from 4 to 10 carbon atoms per molecule and will be liquids under the conditions of the polymerization. Some representative examples of suitable organic solvents include pentane, isooctane, cyclohexane, normal hexane, benzene, toluene, xylene, ethylbenzene, and the like, alone or in admixture.
In the solution polymerizations of this invention, there will normally be from about 5 to about 35 weight percent monomers in the polymerization medium. Such polymerization media are, of course, comprised of the organic solvent, 1,3-butadiene monomer and isoprene monomer. In most cases, it will be preferred for the polymerization medium to contain from 10 to 30 weight percent monomers. It is generally more preferred for the polymerization medium to contain 20 to 25 weight percent monomer.
The monomer charge compositions utilized in the polymerizations of this invention will typically contain from about 5 weight percent to about 95 weight percent isoprene and from about 5 weight percent to about 95 weight percent 1,3-butadiene monomer. It is more typical for the monomer charge composition to contain from about 20 weight percent to about 80 weight percent isoprene and from about 20 weight percent to about 80 weight percent 1,3-butadiene. In most cases, the monomer charge composition will contain from about 30 weight percent to about 70 weight percent isoprene and from about 30 weight percent to about 70 weight percent 1,3-butadiene.
Since the copolymerization of the 1,3-butadiene monomer and isoprene monomer is normally carried out to a high conversion, the ratio of repeat units in the liquid isoprene-butadiene polymer that are derived from isoprene and 1,3-butadiene will be about the same as was employed in the monomer charge composition. Thus, the liquid isoprene-butadiene polymer will normally contain from about 5 weight percent to about 95 weight percent bound isoprene and from about 5 weight percent to about 95 weight percent bound 1,3-butadiene monomer. The liquid isoprene-butadiene polymer will typically contain from about 20 weight percent to about 80 weight percent bound isoprene and from about 20 weight percent to about 80 weight percent bound 1,3-butadiene monomer. In most cases, the liquid isoprene-butadiene polymer will contain from about 30 weight percent to about 70 weight percent bound isoprene and from about 30 weight percent to about 70 weight percent bound 1,3-butadiene monomer.
The liquid isoprene-butadiene polymer of this invention can be made by a batch process or continuously. It will normally be advantageous to synthesize the isoprene-butadiene polymer on a continuous basis. In such a continuous process, the monomers and an organolithium initiator are continuously fed into a reaction vessel or series of reaction vessels. The pressure in the reaction vessel is typically sufficient to maintain a substantially liquid phase under the conditions of the polymerization reaction. The reaction medium will generally be maintained at a temperature which is within the range of about 20xc2x0 C. to about 140xc2x0 C. throughout the copolymerization. The reaction temperature will preferably be within the range of about 40xc2x0 C. to about 100xc2x0 C. during the course of the copolymerization. It is generally most preferred for the reaction medium to be maintained at a temperature which is within the range of about 60xc2x0 C. to 80xc2x0 C. throughout the copolymerization.
The organolithium compounds which can be utilized as initiators in the copolymerizations of this invention include organomonolithium compounds and organo multifunctional lithium compounds. The organo multifunctional lithium compounds will typically be organodilithium compounds or organotrilithium compounds. Some representative examples of suitable multifunctional organolithium compounds include 1,4-dilithiobutane, 1,10-dilithiodecane, 1,20-dilithioeicosane, 1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene, 1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane, 1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane, 1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane, 1,2,4,6-tetralithiocyclohexane, 4,4N-dilithiobiphenyl, and the like.
The organolithium compounds which can be utilized are normally organomonolithium compounds. The organolithium compounds which are preferred can be represented by the formula Rxe2x80x94Li, wherein R represents a hydrocarbyl radical containing from 1 to about 20 carbon atoms. Generally, such monofunctional organolithium compounds will contain from 1 to about 10 carbon atoms. Some representative examples of organolithium compounds which can be employed include methyllithium, ethyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium, tert-octyllithium, n-decyllithium, phenyllithium, 1-napthyllithium, 4-butylphenyllithium, p-tolyllithium, 1-naphthyllithium, 4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium, cyclohexyllithium, 4-butylcyclohexyllithium and 4-cyclohexylbutyllithium. The organolithium initiator will typically be an alkyl lithium compound; such as, n-butyl lithium.
The amount of organolithium initiator employed will be dependent upon the molecular weight which is desired for the liquid isoprene-butadiene polymer being synthesized. As a general rule in all anionic polymerizations, the molecular weight of the polymer produced is inversely proportional to the amount of initiator utilized. Since liquid isoprene-butadiene polymer having a relatively low molecular weight is being synthesized, the amount of initiator employed will be relatively large. As a general rule, from about 0.1 to about 2 phm (parts per hundred parts of monomer by weight) of the organolithium compound will be employed. In most cases, it will be preferred to utilize from about 0.2 to about 1 phm of the organolithium compound with it being most preferred to utilize from about 0.4 phm to 0.6 phm of the organolithium compound. In any case, an amount of organolithium initiator will be selected to result in the production of liquid isoprene-butadiene polymer having a number average molecular weight which is within the range of about 3,000 to about 50,000. The amount of organolithium initiator will preferably be selected to result in the production of liquid isoprene-butadiene polymer having a number average molecular weight which is within the range of about 5,000 to about 30,000. The amount of organolithium initiator will most preferably be selected to result in the production of liquid isoprene-butadiene polymer having a number average molecular weight which is within the range of about 8,000 to about 18,000.
It is critical to carry out the copolymerization of 1,3-butadiene and styrene in the presence of a polar modifier, such as N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine (TMEDA), to attain a high glass transition temperature which is within the range of about xe2x88x9250xc2x0 C. to 20xc2x0 C. It is preferred for the isoprene-butadiene polymer to have a glass transition temperature which is within the range of about xe2x88x9240xc2x0 C. to about 10xc2x0 C. It is generally most preferred for the liquid isoprene-butadiene rubber to have a glass transition temperature which is within the range of about xe2x88x9230xc2x0 C. to about 0xc2x0 C. Normally, a molar ratio of the polar modified to lithium initiator of at least about 0.5:1 will be utilized to attain the desired glass transition temperature. In most cases, a molar ratio of the polar modified to lithium initiator which is within the range of about 0.5:1 to about 20:1 will be utilized. It is normally preferred to use a molar ratio of the polar modified to lithium initiator which is within the range of about 1:1 to about 10:1. It is typically preferred to employ a molar ratio of the polar modified to lithium initiator which is within the range of about 1.4:1 to about 4:1. It is normally more preferred to employ a molar ratio of the polar modified to lithium initiator which is within the range of about 1.6:1 to about 2:1.
Ethers and tertiary amines which act as Lewis bases are representative examples of polar modifiers that can be utilized. Some specific examples of typical polar modifiers include diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, triethylene glycol dimethyl ether, trimethylamine, triethylamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine, N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and the like. Dipiperidinoethane, dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol, dimethyl ether, TMEDA and tetrahydrofuran are representative of highly preferred modifiers. U.S. Pat. No. 4,022,959 describes the use of ethers and tertiary amines as polar modifiers in greater detail.
The isoprene-butadiene polymer produced by the copolymerization is recovered from the organic solvent after the desired monomer conversion has been attained. The isoprene-butadiene polymer can be recovered from the organic solvent by standard techniques. It is normally desirable to precipitate the isoprene-butadiene polymer from the organic solvent by the addition of lower alcohols containing from 1 to about 4 carbon atoms to the polymer solution. Suitable lower alcohols for precipitation of the isoprene-butadiene rubber from the polymer cement include methanol, ethanol, isopropyl alcohol, n-propyl alcohol and t-butyl alcohol. The utilization of lower alcohols to precipitate the liquid isoprene-butadiene rubber from the polymer cement also xe2x80x9ckillsxe2x80x9d the living polymer chains by inactivating lithium end groups. After the isoprene-butadiene polymer is recovered from the organic solvent, steam-stripping can be employed to reduce the level of volatile organic compounds in the polymer.
The repeat units derived from isoprene and 1,3-butadiene that are in the liquid isoprene-butadiene rubber are in an essentially random order. The term xe2x80x9crandomxe2x80x9d as used herein means that the repeat units which are derived from isoprene are well dispersed throughout the polymer and are mixed in with repeat units which are derived from 1,3-butadiene. For purposes of this patent, xe2x80x9crandomxe2x80x9d means that over 60 percent of the isoprene in the IBR is present in blocks of three or less repeat units.
The liquid isoprene-butadiene rubber will exhibit only one glass transition temperature that will be within the range of about xe2x88x9250xc2x0 C. to 20xc2x0 C. It is preferred for the isoprene-butadiene polymer to have its glass transition temperature at a temperature that is within the range of about xe2x88x9240xc2x0 C. to about 10xc2x0 C. It is generally most preferred for the liquid isoprene-butadiene rubber to have its glass transition temperature which is within the range of about xe2x88x9230xc2x0 C. to about 0xc2x0 C. In any case, the liquid isoprene-butadiene polymer of this invention does not exhibit multiple glass transition temperatures. The liquid isoprene-butadiene rubber has a single glass transition temperature that occurs within the range of about xe2x88x9250xc2x0 C. to 20xc2x0 C.
For purposes of this patent application, polymer microstructures are determined by nuclear magnetic resonance spectrometry (NMR). Glass transition temperatures are determined by differential scanning calorimetry at a heating rate of 10xc2x0 C. per minute and molecular weights are determined by gel permeation chromatography (GPC).
The liquid isoprene-butadiene rubber of this invention is particularly valuable for use in making treads for high performance automobile tires. Such high performance tires will normally include a generally toroidal-shaped carcass-with an outer circumferential tread, two spaced beads, at least one ply extending from bead to bead and sidewalls extending radially from and connecting said tread to said beads. The tread of such tires is, of course, adapted to be ground-contacting. The treads of this invention are comprised of (a) a rubbery elastomer, (b) about 20 phr (parts by weight per 100 parts by weight of the rubbery elastomer) to about 80 phr of an aromatic process oil, (c) about 35 phr to about 130 phr of a filler and (d) about 4 phr to about 40 phr of a liquid isoprene-butadiene polymer of this invention. Such treads provide tires with significantly improved dry traction without sacrificing durability.
In the tire tread formulations of this invention, the liquid isoprene-butadiene polymer is essentially substituted for a portion of the aromatic processing oil normally included in the tread compound. The liquid isoprene-butadiene polymer will be added to the tread formulation in an amount that is within the range of about 4 phr to about 40 phr. In most cases, it is preferred for the liquid isoprene-butadiene polymer to be present in the tread compound in an amount which is within the range of about 5 phr to about 25 phr. The amount of aromatic process oil added to the tread compound will typically be within the range of about 20 phr to about 80 phr. In high performance passenger tire applications, the oil will typically be added in an amount that is within the range of about 20 phr to about 50 phr. In high performance passenger tire applications, the oil will more typically be added in an amount that is within the range of about 30 phr to about 45 phr. In race tire applications, the oil will typically be added in an amount that is within the range of about 50 phr to about 80 phr. In race tire applications, the oil will more typically be added in an amount that is within the range of about 55 phr to about 70 phr.
The rubbery elastomer employed in the tread compound can be any rubber or blend of rubbers that can be used in manufacturing high performance tire treads. For instance, the rubbery elastomer can be natural rubber, emulsion styrene-butadiene rubber, solution styrene-butadiene rubber, styrene-isoprene-butadiene rubber or a mixture thereof Such blends can also contain 3,4-polyisoprene, trans-1,4-polybutadiene or cis-1,4-polybutadiene rubber. In cases where 3,4-polyisoprene, trans-1,4-polybutadiene or cis-1,4-polybutadiene rubber are included in the tread compound, they will generally be employed at a level of less than about 40 phr. For instance, the 3,4-polyisoprene, trans-1,4-polybutadiene or the cis-1,4-polybutadiene can be employed in an amount which is within the range of about 5 phr to about 35 phr. The 3,4-polyisoprene which can be utilized will normally have a 3,4-microstructure content of 55 percent to 80 percent as determined by NMR spectroscopy. The 3,4-polyisoprene will, accordingly, have a cis-1,4-microstructure content which is within the range of 20 percent to 45 percent. The 3,4-polyisoprene will also normally have a glass transition temperature from xe2x88x9225xc2x0 C. to 10xc2x0 C. as determined by differential scanning calorimetry at a heating rate of 10xc2x0 C./minute. The cis-1,4-polybutadiene that can be employed in the tread compound will typically have a cis-1,4-microstructure content of at least about 96 percent as determined by NMR spectroscopy.
In many cases, it is preferred for the rubbery elastomer in the tread compound to be styrene-butadiene rubber. For instance, the tread compound can be made using only styrene-butadiene rubber as the rubbery elastomer. However, the rubbery elastomer can also be a blend of styrene-butadiene rubber with natural rubber, 3,4-polyisoprene, cis-1,4-polybutadiene, trans-1,4-polybutadiene or styrene-isoprene-butadiene rubber. In cases where the styrene-butadiene rubber is blended with another rubbery polymer, the other rubbery polymer will normally be added in an amount that is within the range of about 5 phr to about 40 phr.
The tread compound will also contain at least one filler, such as carbon black and/or silica. Clays and/or talc can be included in the filler to reduce cost. The filler will normally be present in an amount which is within the range of about 35 phr to about 130 phr. In high performance passenger tire applications, the filler will normally be present in an amount which is within the range of about 35 phr to about 70 phr. In race tire applications, the filler will normally be present in an amount that is within the range of about 70 phr to about 130 phr. The tread compound can also contain one or more resins, such as, coumarone-indene resin. The resin will normally be added in an amount that is within the range of about 5 phr to about 60 phr in race tire applications. In passenger tire applications, the resin will typically be added in an amount that is within the range of 0 phr to about 20 phr. In passenger tire applications, it is typically preferred for the tread compound not to contain any resin.
The liquid isoprene-butadiene polymer containing blends of this invention can be compounded utilizing conventional ingredients and standard techniques. For instance, the liquid isoprene-butadiene polymer containing rubber compound will typically also include sulfur, accelerators, waxes, scorch inhibiting agents and processing aids. In most cases, the tread rubber formulation will be compounded with sulfur and/or a sulfur containing compound, at least one accelerator, at least one antidegradant, at least one processing oil, zinc oxide, optionally a tackifier resin, optionally a reinforcing resin, optionally one or more fatty acids, optionally a peptizer and optionally one or more scorch inhibiting agents. Such blends will normally contain from about 0.5 to 5 phr (parts per hundred parts of rubber by weight) of sulfur and/or a sulfur containing compound with 1 phr to 2.5 phr being preferred. It may be desirable to utilize insoluble sulfur in cases where bloom is a problem.
The blend will also normally include from 0.1 to 2.5 phr of at least one accelerator with 0.2 to 1.5 phr being preferred. Antidegradants, such as antioxidants and antiozonants, will generally be included in the blend in amounts ranging from 0.25 to 10 phr with amounts in the range of 1 to 5 phr being preferred. The liquid isoprene-butadiene polymer containing blends of this invention will also normally contain from 0.5 to 10 phr of zinc oxide with 1 to 5 phr being preferred. These blends can optionally contain from 0 to 30 phr of tackifier resins, 0 to 10 phr of reinforcing resins, 1 to 10 phr of fatty acids, 0 to 2.5 phr of peptizers and 0 to 1 phr of scorch inhibiting agents.
The liquid isoprene-butadiene rubber containing tread rubber compounds of this invention can be used in tire treads in conjunction with ordinary tire manufacturing techniques. In other words, tires can be built utilizing standard procedures with the liquid isoprene-butadiene rubber simply being substituted for a portion of the aromatic oil normally included in the tread rubber compound. After the tire has been built, it can be vulcanized using a normal tire cure cycle. Tires made in accordance with this invention can be cured over a wide temperature range. However, it is generally preferred for the tires of this invention to be cured at a temperature ranging from about 132xc2x0 C. (270xc2x0 F.) to about 166xc2x0 C. (330xc2x0 F.).